Research has shown that the mere categorization of persons into groups is sufficient to promote intergroup bias--favoritism of the ingroup at the expense of the outgroup. These findings may be explained in terms of a change in the person's perception of the social situation once others are categorized into groups. Ingroup-outgroup bias is hypothesized to be a function of two judgmental processes in the individual. (1) Perception of an outgroup arouses a complementary ingroup and the expectation that the outgroup is different from one's ingroup, and therefore warrants differential treatment. (2) The greater the unity of a person with his ingroup, the stronger is the resulting ingroup favoritism. Given this anaysis of ingroup-outgroup bias, several experiments are proposed to test methods of reducing ingroup favoritism. First, ingroup bias should not occur if the outgroup is not perceived as a single entity but rater as a collection of individuals. Second, ingroup bias should be reduced if the situation is restructured so that the outgroup is perceived to be more like the ingroup or even a part of the ingroup. Finally, ingroup-outgroup bias may be ameliorated by partitioning the person from the ingroup through decreased contact with the ingroup. These hyptheses will be examined in a series of laboratory experiments; if successful, findings will be applied to the sudy of ongoing groups in field settings.